The main reason why a defendant might be provided with a witness' statement is so that the defendant has a fair chance to prepare to rebut anything negative that is in it. The two major ways of doing this are cross-examination of the witness and the calling of the defendant's own witnesses to rebut the allegations. A Congressional hearing is a whole different sort of animal, and witnesses don't participate in this way.

Edit: A couple posts were made while I was typing. Apologies for any confusion.